The invention relates to a rotor structure for a motor having a built-in type permanent magnet, and more particularly, to a rotor structure for a motor having a built-in type permanent magnet that possesses an approximately-sinusoidal distribution of magnetic flux in the air-gap which results in a significant reduction in both the cogging torque and the rotational vibration of motor.
FIG. 5A is an exploded view showing the disposition between a built-in type rotor and the stator of the prior art while FIG. 5B is an end view showing the disposition between the built-in type rotor and the stator of the prior art. As shown in FIG. 5A and FIG. 5B, the rotor (100) includes a shaft hole (103) for shaft assembly and a multiplicity of axial magnet slots (102) for containing permanent magnets (not shown in the Figure). In order to reduce the cogging torque, the conventional motor employs in its stator (200) a plurality of skew slot structures (104) with a skew angle xcex1, or in accompaniment with the number of stator""s slots, employs a polar plane developed in an angle having a non-integer slot number. But the former makes the winding work of the motor hard to implement, and the cost is high and the magnetic flux distribution of the air-gap is not easy to control. While the latter makes the developed angle of the polar plane relatively small, which directly reduces the output of the motor, thereby, is not apt to apply in stator having relatively low number of slots.
FIG. 6 is a diagram of magnetic flux distribution of the air-gap of FIG. 5A and FIG. 5B of the prior art. As shown in FIG. 6, the magnetic flux distribution of the air-gap is a trapezoidal distribution which can result in a relatively large cogging torque.
Therefore, the disadvantages of the prior art can be listed as follows:
(a) The cogging torque of the motor is relatively large.
(b) The developed angle of the polar plane of the permanent magnet is relatively small.
(c) The permanent magnet is apt to generate magnetic flux leakage.
(d) Skew slots that create difficulty in winding work of the stator of the motor is needed in order to reduce the cogging torque.
In light of the disadvantages of the prior art, the invention provides a rotor structure that can reduce the cogging torque, weaken the armature reaction, as well as prevent the magnetic flux leakage.
Therefore, it is an objective of the invention to provide a circular outer edge of rotor structure for a motor having built-in type permanent magnet that employs a petal-shaped circular pole-plane that is eccentric from the center of the rotor. This rotor structure of the invention can modify the trapezoidal magnetic flux distribution of the air-gap into a approximately-sinusoidal distribution that can improve the shortcoming of being excessive cogging torque of the rotor and simplify the winding work of the motor. The implementation of the invention is nothing to do with the slot number of the stator. The outer edge of the petal-shaped circular arc contour also provides a relatively large resistance to the armature reaction. In addition, due to the precise servo control, the current of the motor is a sinusoidal current. The fact that the magnetic flux of the air-gap of the invention is a approximately-sinusoidal distribution can obtain an uniform output torque, and is quite adequate for the application in precision servo control.
Another objective of the invention is to provide axial empty slots positioned between the shaft hole and the corners where the adjacent magnet slots meet. These axial empty slots can not only prevent the permanent magnet between poles from magnetic leaking but can also directly reduce the rotational inertia of the rotor of the motor. In the meantime, the axial empty slots can directly reduce the rotational inertia of the rotor, shorten the starting time, and reduce the no-load current of the motor.
To achieve the above-mentioned objectives, the invention provides a rotor structure for a motor having built-in type permanent magnet. The rotor includes a shaft hole for installing a shaft, and a multiplicity of axial magnet slots for containing permanent magnet. The invention essentially forms a petal-shaped convex polar portion having an exterior circular arc on the outer edge of the axial magnet slot where the radius of the circular arc is smaller than the radius of the rotor. The petal-shaped convex polar portion provides an approximately sinusoidal distribution of air-gap magnetic flux that can improve the shortcoming of being too large in cogging torque of the motor, weaken the armature reaction of the motor, and prevent the permanent magnet from being demagnetized. In addition, an axial empty slot is also provided between the shaft hole and the corners where the adjacent magnet slots approach such that the magnetic flux leakage can be prevented and the weight of the rotor can be reduced too.